The invention relates to a grid casting mold for the casting of lead grids for accumulators comprising two plates which can be placed against one another with a casting profile or mold cavity complementary to the lead grid which is to be cast being provided in their confronting mold surfaces, and also relates to a method for its manufacture.
For the manufacture of lead grids for accumulators in gravity castings use is generally made of such grid casting molds of steel and grey cast iron. The geometry of the lead grid to be manufactured is introduced into the base body by chip forming machining with a certain extra clearance for the thermally insulating layer which is to be applied in a pressurised air spraying process and which consists of cork flour with grain sizes of less than 200 mesh which are dispersed with a binder, for example water glass or Relatin (a registered trade mark of the German Henkel company), i.e. carboxymethyl celluloses. The suspension of cork flour and binder is applied onto the mold surface of the base body which is already provided with the casting profile by means of a spray device. The layer thickness of the thermally insulating layer amounts to ca. 0.15 mm. The thermally insulating layer simultaneously satisfies the function of a mold parting or release agent.
The tempering of the mold which is necessary in order to obtain the corresponding manufacturing parameters takes place through cooling channels which are provided in the base body through which a suitable fluid is directed. At the start of work the casting mold is brought up to the desired temperature level by appropriate auxiliary heating means. The lead grid is then cast and, after a certain time, the mold is cooled by the introduction of a cooling agent into the cooling passages. The thermal insulating layer thereby prevents too rapid solidification of the lead melt.
A problem with the known grid casting mold lies in the fact that the lifetime of the thermally insulating layer which has been applied is restricted and in the fact that the weight of the lead grid which is manufactured can change due to continuous wear. This requires, also in dependence on the type of product being produced, a repair of the insulating layer or indeed the removal and complete new application of the latter daily.
The cause of the wear or the die-coating material is the high thermal loading due to the operating temperature of the mold and the melt temperature, and also the flow speed of the liquid lead alloy.
The casting molds are built up in accordance with the geometry of the grid from two plates of for example 30 to 50 mm thickness. Cast iron with spherical graphite or, for less demanding molds, the more favorably priced and easier to procure grey cast iron are also suitable as material for the casting mold.
A method of manufacturing an apparatus for the casting of lead grids for electrical accumulator plates is already known (German laying open prints 35 29 725 A1 and 36 03 657 A1) in which the apparatus contains ceramic material the surface of which forms the casting mold and which stands in direct contact with the molten material entering into the die, with the ceramic material having a high porosity and being manufactured by flame spraying. The flame-sprayed ceramic layer is matched by mechanical material removing machining to the exact contour of the cast part. An intermediate layer located between the base body and the metal oxide layer serves as a bond promotor. The disadvantage of the known method is, on the one hand, the lack of thermal insulation and, on the other hand, the requirement for subsequent mechanical machining of the hard ceramic layer.
Furthermore a casting mold consisting of two mold halves for the manufacture of grid plates for lead accumulators is already known which are inserted as a negative form into the respective parts of an outer metallic mold carrier (EP 219 610 B1), with the casting mold being formed from a highly porous mat of microfibers. This prior known highly porous fiber mat mold requires no further surface treatment with a separating agent or insulating material for the metal casting. The micro fleece molds are not free from wear, they can however be rapidly exchanged for a new mold pair. The fiber fleece molds are thus consciously manufactured so that they only have a restricted working life.